This invention relates to acoustic surface wave devices. Such devices are well known and commonly used for filters, delay lines, matched filters and pulse expansion and compression devices.
Acoustic surface waves may be launched on a piezoelectric substrate by applying electric signals to a suitable transducer. Likewise a transducer may be used to convert such acoustic surface waves into electrical signals. Acoustic surface waves have a propagation velocity which is much lower than the propagation velocity of electrical signals. This property is useful in utilization of acoustic surface wave devices as filters and delay lines.
The most commonly used acoustic surface wave transducer is the "interdigital" transducer which comprises interleaved arrays of conductive fingers disposed on one surface of the piezoelectric substrate. The interleaved arrays usually have a fundamental periodicity at which the pattern of conductive fingers is repeated which is approximately equal to one wavelength at the operating frequency of the transducer. Typical interdigital transducers are described in U.S. Pat. Nos. 3,581,248 (DeVries) and 3,727,155 (DeVries) which show respectively interdigital transducer with two and four conductive fingers per acoustic wavelength. Another transducer having three conductive fingers per acoustic wavelength is described in the copending application of C. F. Vasile, Ser. NO. 453,617 filed Mar. 22, 1974, entitled "Surface Wave Transduer having Three Conductive Fingers per Acoustic Wavelength," which is assigned to the same assignee as the present application.
In an article entitled "Excitation of Elastic Surface Waves by Spatial Harmonics of Interdigital Transducers," by Helge Engan which was published in IEEE Transactions on Electronic Devices, Dec. 1969, there is described the coupling effect of an interdigital transducer at the harmonics of the frequency at which the fundamental periodicity corresponds to one wavelength for a transducer having two conductive fingers per acoustic wavelength. In that article it is shown that a transducer can be made to operate efficiently at certain harmonics of the frequency corresponding to the fundamental periodicity of the transducer.
It is therefore an object of the present invention to provide an acoustic surface wave device having surface wave transducers coupled to a piezoelectric substrate by harmonic coupling.
It is a further object of the present invention to provide such a device for operation in a selected range of frequencies having two transducers wherein each transducer has a different fundamental periodicity and has harmonic coupling in the selected range of frequencies.
In accordance with the present invention, there is provided an acoustic surface wave device which operates over a selected range of frequencies. This device includes a piezoelectric substrate and a first surface wave transducer having a first selected fundamental periodicity and having a first set of harmonic responses only one of which lies substantially within said selected range of frequencies. The device further includes a second surface wave transducer having a second selected fundamental periodicity, differing from said first selected fundamental perioidicity, and having a second set of harmonic responses, only one of which overlaps substantially with any of the harmonic responses of said first transducer, said overlap occurring within said selected range of frequencies.